Method of acoustic measurement and apparatus therefor



Feb. 27, 1945.

.L. G. ABRAHAM ETAL METHOD OF ACOUSTIC MEASUREMENT AND APPARATUS THEREFOR Filed NOV. 17, 1942 E (was) FIG. 3

L. a. ABRAHAM 0. MITCHELL IN [/5 N TORS 3mm T7'ORNEV I l ,4 s MICROPHONE-SPEAKER SEPARAHON Patented Feb. 27, 1945 UNITED STATES. PATENT OFFICE METHOD OF ACOUSTIC MEASUREMENT AND APPARATUS THEREFOR Leonard G. Abraham, v

Mitchell, Bound Brook, N. J., assignors to Bell Telephone Laboratories,

Madison, and Doren Incorporated, New

York, N. Y., a corporation of New York Application November 17, 1942, Serial No. 465,842

present invention the desired acoustic assessment 9 Claims. This invention relates to means for determining the acoustic character of a room or other enclosure, particularly with reference to measuring the echoes evoked in such enclosure by sounds of short duration generated therein. The invention resides in the coaction of its constituent apparatus elements and in the method of acoustic measurement to which the coacting elements are adapted.

An object of the invention is to facilitate the making of acoustic determinations.

Another object of the invention is to provide a method and apparatus whereby the acoustics of rooms or other enclosures may be rapidly evaluated. I Since the disturbing efiect'of echoes depends on their magnitude relative to the original sound, another object of the invention is to provide a method and apparatus with which the echoes evoked by sound pulses in the enclosure may be measured as related to the sound pulses themselves.

Finally, since the disturbing effect Of echoes tends to increase with their delay, it is an object of the invention to provide an echo measuring apparatus including weighting means whereby progressively later echoes are assigned progressively greater weights.

A specific problem to which the invention may be applied is the assessment of the suitability of a room for loud-speaking telephoneconferences. For the sake of illustration the invention will be described with reference to this application, but

it will be clear that no limitation is thereby implied.

For such purposes, each room whose occupants desire to confer by telephone with the occupants of other rooms is provided with a microphone and a loud-speaker. These instruments must be so located with respect to eachother that singing is prevented and that echoes from walls or other acoustic reflectors do not confuse the intelligible transmission of speech uttered in the room. It is found by experience that echoes later than the first are progressively more disturbing, from which it appears that the subjective judgment of the suitability of a room involves a spontaneous although perhaps unrecognized weighting of the echoes wherein greater and greater weights are assigned to the successively later echo components. Subjective J'udgment's'of this kind are reliable when-made by a. trained crew of talkers and listeners but the training of 'such crews is expensive and their observations time consuming. By the use of the is made rapidly and impersonally.

The invention will be understood and objects other than those explicitly stated will become' apparent from the following description read in connection with the accompanying which:

Fig. 1 is acircuit diagram of the apparatus of Referring to Fig. 1, 80-volt battery Ill charges.

condenser ll through resistor 12. Condenser Ii is shunted by a circuit comprising in series discharge tube T1 and potentiometer l3. T1 is a Thyratron," for example, the Western Electric 313-0, of which the breakdown voltage is '70 volts or somewhat less than that of battery 10. When condenser ll acquires a charge of volts, tube T1 becomes conducting and condenser H discharges through T1 and potentiometer i3. It is obvious that any other Thyratronfmay be substituted for T1, it being required onlythat the voltage of battery Ill be somewhat above its breakdown point. Condenser H and resistor 12 together determine the frequency of recurrence of the breakdown of tube T1. For the present specific application, condenser H is 1 microfarad capacitance and resistor l2 of 10 megohms resist- I ance, corresponding to an interval of one-half second between successive discharges of condenser l I. Obviously, this interval may be varied to suit the particular circumstances in which the invention is to be used. Conveniently, tap H on resistor i2 is provided for such variation.

The required poling of battery i0 is shown in Fig. 1. Positive battery terminal is connected through switch S to one terminal of condenser i I, to ground and to the end of potentiometer I3 remote from tube T1. "With this connection, in

the dis'chargeof condenser ll a'sharp voltage pulse appears across potentiometer IS, the end thereof remote from ground becoming negative to ground. Shunting potentiometer i3 is the series connection of resistor l5 and potentiometer l6,'the latter element being itself shunted by drawing, in

-progressively annulled.

- former 2! toweighting circuit any purpose to provide an amplification characteristic for the circuit of tube T: having a droop at low and at high frequencies, this may be acoomplished by connecting condenser 43 in parallel with the primary of transformer 32. In a particular case where transformer 32 was the Westem Electric 132--A, a capacitance of 0.007 microfarad for condenser ll provided the desired fr..-

At any desired distance from loud-speaker is placed microphone 2i which is therefore capable of receiving sharp sound pulses from loudspeaker 20 and such echoes of that pulse as may be returned by sound reflecting surfaces in the neighborhood. The output of microphone 20 after transmission through a circuit later to be described furnishes the information sought in the study of the enclosure indicated by the dashed line of Fig. 1, wherein the apparatus is installed. It is optional to install the entireapparatus or only loud-speaker 20 and microphone 2| in the enclosure to be studied.

It was mentioned above that potentiometer i0 is shunted by condenser ll, the capacitance ofwhich is vl microfarad. Condenser I1 is itself rapidly charged through resistor I! by the discharge current from condenser II and at once begins slowly to discharge through the parallel paths II, I! and II. The rate of discharge of condenser II is determined by the resistances of potentiometers i3 and ii, that or resistor ll being negligible in comparison. Suitable values for the resistance of potentiometers it and ii are 500,000 ohms and 300,000 ohms, respectively, the latter value being obtained by adjustment of a tap on A potentiometer it. With these resistances, condenser discharges through approximately 200,-

000ohms, giving a time constant of 0.2 second,

insuring that condenser l1 shall be completely discharged before it recharges as the result of another breakdown of tube T1. In the comparatively slow discharge of condenser I! the voltage negative to ground across potentiometer II is The output of microphone 2i is amplified by amplifier 22, and is transmitted through trans 24. the control of which is presentlyto be described, thence through transformer to potentiometer 20, a selected portion of which forms the control grid to ground circuit of vacuum tube Ta. Tube T: may properly be a pentode, for example. the Western Electric 310-A. Grounded in common with the Po itive terminal of battery II is the negative terminal of battery II, of voltage approximately 135 volts. Battery supplies from its positive terminal through inductance II and the primary winding 'of transformer 82 a suitable voltage to-anode a of tube Ts. In this'tubethe electrodes are conveniently connected. the suppressor grid 84 in cathode 3|, screen grid ll through inductance ii to the positive terminal of battery 30, cathode II to ground through biasing resistor 81 by- -passed by condenser 80, and control grid II to a point on potentiometer 20 selected by tap 21. Suitably inductance II is so henrles; resistor u, see ohms; 1

condenser 28, 25-microfarads.

Tube '1': is therefore a straightforward ampliwelghting circuit 24' from the amplified output of 'inicrophone 2|. 'I'ap 21 of potentiometer 2| quency characteristic.

The output of tube '1: is transmitted through transformer 32 to rectifler 40, which may be of any convenient design. The steady voltage derived from the rectifier 40 charges condenser 4|, which slowly discharges through resistor 42 in series with ammeter I0. Appropriately, '500 mic'rofarads is the capacitance of condenser 4i and 50,000 ohms the resistance of resistor 42. The time constant of condenser 4| and resistor 42 is then 25 seconds, great enough to insure that the sound pulses emitted by loudspeaker 20 at a recurrence frequency of 2 cycles per second shall after a short time be manifested as a steady deflection on meter 50.

With the circuit described, a sharp sound pulse lasting about 10 milliseconds is produced by a loud-speaker 20. This pulse with all its following echoes wil be measured by meter I0 provided the weighting circuit 24 is left uncontrolled. As previously mentioned, it is desired as part of the observing method to measure the echoes alone, giving progressively greater weight to progressively later echo components. This is accomplished by weighting circuit which is preferably composed of thermosensitive elements such as Western Electric 19--A varistors, described in Patent 2,106,344 to J. R. Flegal et al., January 25, 1938. Such circuit elements have an impedance to alternating current which is lower the greater the direct current flowing through them. With in the rectangle indicated by numeral 2!, four l9A varistors V1, Vs, Vain-11d V4 are shown in tandem pairs shunted terminally and centrally by 300-ohm resistors R1, R: and Ra.

In the present invention the controlling direct current is that flowing in the input circuit of tube Ta, suitably the Western Electric 310-A. Cathode and suppressor grid 45 of T: are grounded and so connected to the negative terminal of battery 30. Positive terminal of battery 30 is connected through resistor 41 and milliammeter 1 tube '1: between the upper and lower varistor paths. Between cathode H and milliamrneter 48 are connected resistor ll and potentiometer I in series, which-are conveniently of resistances 2,000 ohms and 10,000 ohms, respectively. The

tap on potentiometer I. is adjusted to provide a tube '1: read on milliammeter 40 is controlled by the voltage of control grid ll, namely, the-voltage fler-femthrough transformers I3 and ind I between tap II and ground. Condenser ll 0! about 4 microfarads capacitance, together with 1,000-ohm resistor", serves to-fllter the current from battery II to anode 40. Similarly, 4-microfarad condenser ll cooperates with inductance II to filter the anode currentof tube T1.

The dischargeof cmdenler' ll-results in a' shall becomevolts negative at the commencement of the voltage pulse across potentiometer Hi. This negative voltage on grid 58 sufllces to drive tube T3 to cut oil and vanishes comparatively slowly with discharge of condenserv i'l, whereby the conductivity of tube T3 is gradually reestablished. Accordingly, at the outset of the discharge on condenser H the anode current of tube Ta is sharply reduced and gradually recovers its normal value. This anode, current flowing through weighting circuit 24 in the direction indicated by the arrow confers on circuit 24 an alternatingcurrent impedance initially high but gradually reduced to the value corresponding to the anode current of tube T: when the voltage on control grid 58 is zero.

The recovery of conductivity by tube T3 and of transmission by circuit 24 is substantially complete within the half second elapsing between successive discharges of condenser H. The 10-. millisecond direct sound pulse from loud-speaker 21! is received by microphone 2| but is substantially completely excluded from affecting meter 50, whereas the successive echoes following this pulse are permitted by the gradually restored transmission of circuit 24 to contribute increasingly to the reading of meter 50. This effect is produced at each recurrence of a sound pulse and is integrated to cause a steady reading of meter 50.

The curve of Fig. 2A is plotted from measurements of over-all gain between the input to weighting circuit 24 and the input to rectifier '40 for steady negative voltages on grid 58 varying from -5 volts to zero. As a first approxi-' mation, the negative pulse across potentiometer l3 and so the voltage of control grid 58 may be- Assuming an taken to vanish exponentially. exponential variation of the voltage of control grid from 5 volts to zero, the voltage E of grid 58 may be expressed as follows:

z E: 5e RC where R=200,000 ohms C=1 microfarad t=the time in seconds elapsed since the onset of the negative voltage pulse across the potentiometer I3.

The value of E computed together with the First, tap 59 is set at the grounded end of potentiometer i3.- The circuit is then set in operation by closing the switch S. No negative voltage then appears on the control grid of tube T3 and meter 50 reads, after a short interval, a steady deflection corresponding to the direct sound pulse and all its echoeswithout discrimination. Potentiometer 25 is then so tapped as to cause meter ill to read 100, or any convenient arbitrary value. The echoes are ordinarily of small magnitude in comparison with the direct sound so that the reading of the meter 50 may be considered correspondent to the direct sound alone. Next, the tap on potentiometer i3 is set to introduce weighting of the echoes. The read ing of meter 50 will now decrease to a new value, in general too small for accurate reading, so that it is convenient to increase the setting of tap 21 on the potentiometer 25 by some predetermined amount. It is then necessary to determine the optimumsetting of tap 59 and the optimum fraction of potentiometer l6 which shall shunt condenser i1. Various procedures to this end may be found practically useful, one of which will nowbe described.

Having altered the setting of tap 21 on potentiometer 25 by a known amount suilicient to aiford a convenient reading on meter 50, one may repeat the steps above described for a different microphone loud-speaker separation and set tap 59 so that one" obtains the greatest percentage sponding to the unweighted echoes.

difference between the readings of the weighted echoes by meter 50 inv the two separations of The adjustment microphone and loud-speaker. of potentiometer I5 is chosen in the same way. It is desirable thereafter to leave undisturbed the optimum settings thus established.

For each microphone loud-speaker separation to be observed, it is appropriate to repeat the calibration of the circuit by causing meter 50 to read a predetermined current value corre- Following this calibration, one reads the weighted echoes alone, having madefor this reading alteration in scale by resetting tap 21 as alread found convenient. The reading of the meter for the weighted echoes is now a, measure of the acoustic excellence of the room. Repeating the observations above'described at various microphone loudspeaker separations, it is thus possible to establish a standard reading for the weighted echoes which corresponds'to the maximum microphone loud-speaker separation judged acceptable by the' human listeners. Once this standard is established, the testing crew may be replaced by the apparatus of the invention and the desired information rapidly obtained.

In Fig. 3 are plotted curves showing for several rooms the weighted echoes measured by the apparatus of Fig. 1 used as just described for various separations of microphone and loud speaker. It will be noted that the reading of 12 rnicroamperes on meter 50 corresponds in rooms a, B and C to the microphone loud-speaker seprration of 1, 2 and 3 feet, respectively. From Judgmentsby an observing crew, the maximum acceptable separations were for the same room /2, 2 and 3 to 4 feet, respectively.

The apparatus of the invention may be briefly described as comprising a generator of recurrent sharp voltage pulses translated by a loud-speaker into corresponding sound pulses which are thereupon retranslated by a microphone into .electrical currents suppliedto an integrating meter through a circuit, the transmission of which varies with time in a manner controlled by the comparatively slow discharge of a' condenser initially charged at each recurrence of the generated voltage pulse. The separate constituent elements of the apparatus described may each be replaced by one of any other type capable of the same function as may be preferred by one skilled in the art without ceasing to use the method of the invention, which consists in measuring with progressively increasing weights the successively later-echoes evoked by a sound pulse and comparing the integrated measurement of the echoes with a measurement of the .echo evoking pulse, thereby evaluating the disturbing effect of the echoes 6. Means for evaluating the acoustic quality of an enclosure including a generating circuit producing sound pulses evoking echoes in said which they are produced.

While the invention has been described with reference to a particular application, obviously it is generally applicable to the evaluation of the acoustic quality of any enclosure. For this purpose it is desirable to choose a fixed separation of microphone and'loud-speaker and'make measurements of the kind described in enclosures known to be acoustically acceptable. By such measurements one may establish a standard reading of meter 50 for the weighted echoes. Comparing with this standard similar measurements made with the chosen microphone-loudspeaker separation in any enclosure, one is enabled to assess the acoustic quality of the enclosures measured and determine the need for treatment to render it acoustically satisfactory.

What is claimed is:

l. The method of evaluating the acoustic quality of an enclosure which consists in generating a series of recurrent sound pulses in said enclosure to evoke echoes therein, receiving and measuring said echoes with the assignment of progressively increasing weighting'to successively later components of said echoes, and comparing the result of said measurement with a previously established standard.

2. The method of evaluating the acoustic quality of an enclosure which comprises a first step of generating-in said enclosure a first series of recurrent sound pulses evoking echoes therein and measuring with equal weighting the components of said first series of sound pulses and echoes, a second step of generating in said en closure a second series of recurrent sound pulses an electrical impulse receiving circuit, an acoustic link between said circuits, and means tovary the transmission efficiency of said receiving circuit with respect to time under the control of said generating circuit.

4. In an apparatus for evaluating the disturbing effect of acoustic echoes, means for generating a sound. pulse evoking echoes, means for receiving and measuring said pulse together with the echoes thereby evoked, and means ad- Justably associated with said generating means for controlling as a function of time the efliciency of said receiving and measuring means.

5. Means for evaluating the acoustic quality of an enclosure including a generating circuit producing sound pulses evoking echoes in said enclosure, a receiving circuit for measuring said echoes,. and a disabling circuit excited by said enclosure, a receiving circuit for measuring said echoes, said receiving circuit including an array of thermosensitive elements, and a disabling circuit excited b said generating circuit and controlling as a. function of time the impedance to alternating current of said array.

7. Means for evaluating the acoustic quality of an enclosure, including a generating circuit producing sound pulses evoking a plurality of successive'echoes in said enclosure, a receiving circuit for measuring said pulses and said echoes, and a disabling circuit excited by said generating circuit to annul substantially completely the efiiciency of said receiving circuit during the reception of said pulses and to restore said eiiiciency progressively with time during the reception of said echoes.

8. In an apparatus for evaluating the acoustic quality ,of an enclosure bounded at least in part uring said sound pulses and echoes including a microphone, a weighting circuit of controllable transmission associated with said microphone and an integrating measuring circuit, and means ad- Justably associated with said generating means for controlling as a function of time the transmission of saidweighting circuit. i

9. In an apparatus for the measurement of the trains of echoes evoked by recurrent sound pulses. a generator of. recurrent voltage pulses, an output circuit for said generator, a loudspeaker coupled to said output circuit for translating said voltage pulses into sound pulses, a shunt circuit connected in parallel with said output circuit, said shunt circuit comprising a condenser in series'with a potentiometer, said condenser being itself shunted by an adjustable resistor, a vacuum tube having at least a control grid, a cathode and an anode, power supply for said tube, an adjustable tap on said potentiometer connected to said control grid, a transmission path in series between the anode and the cathode of said tube, said path being compoud of two parallel banks of thermosensitive elements in tandem connection joined by resistors at a plurality of corresponding points, a first transformer having primary and secondary windings of which the secondary winding joins at one end said banks and is connected at its mid-point to said cathode, a second transformer having primm; circuit and the secondary winding of said generatingcircuit and controlling as a function second transformer, and an integrating measur-' ing circuit coupled to said rectifying circuit.

' woman G. ABRAHAM.

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